The present invention relates to rotary regenerative heat exchangers generally used as air preheaters and more specifically to rotary regenerative heat exchangers having a semi-modular rotor construction.
A rotary regenerative heat exchanger is employed to transfer heat from one hot gas stream, such as a hot flue gas stream, to another cold gas stream, such as combustion air. The rotor contains a mass of heat absorbent material which first rotates through a passageway for the hot gas stream where heat is absorbed by the heat absorbent material. As the rotor continues to turn, the heated absorbent material enters the passageway for the cold gas stream where the heat is transferred from the absorbent material to the cold gas stream.
In a traditional rotary heat exchanger, the cylindrical rotor is disposed on a vertical central rotor post and divided into a plurality of sector-shaped compartments by a plurality of radial partitions, referred to as diaphragms, extending from the rotor post to the outer peripheral shell of the rotor. These sector-shaped compartments are loaded with modular heat exchange baskets which contain the mass of heat absorbent material commonly formed of stacked plate-like elements.
The rotors of such heat exchangers are either formed as non-modular rotors or as modular rotors. The non-modular rotors comprise a series of diaphragm plates each attached to the rotor post and extending out to the rotor shell thereby dividing the rotor into sectors. Further, each sector is divided into a number of compartments by stay plates extending between the diaphragms at spaced intervals. The modular heat exchange baskets are then loaded axially into these compartments from the top end. The non-modular rotors are field labor intensive because all of the heat transfer surface is field installed. Further, more structural welds are required to field assemble non-modular rotors. The result is more total time to field install the heat exchanger rotor structure.
Modular rotors are composed of a series of shop-assembled sector modules which are then field-assembled into a complete rotor. Each sector module has a diaphragm plate on each side with these two diaphragms being joined by stay plates. The heat transfer surface of the sector modules is also shop installed. When these modules are assembled into a rotor in the field, the diaphragm plates of adjacent modules are joined together to form a double plated diaphragm. Although the modular rotors require less time to field-install than non-modular rotors, they require twice as many individual diaphragm plates which take up gas flow area and allow less heat transfer area for the same size rotor and post diameter. Also, they are component intensive because of all the parts necessary to pin the adjacent modules to each other at diaphragm locations.
Most modular and non-modular rotor designs contain stay plates as previously described. The stay plates reinforce the rotor structure and support the baskets. Because the baskets are inserted axially and must fit in the stay plate compartments, the baskets must be undersized for easy installation and removal. Undersizing involves providing a gap around the perimeter of each basket. This reduces the free area of the basket available for heat transfer flow and creates flow bypass gaps around the baskets. The result is decreased air preheater efficiency and the selection of larger air preheaters for any particular performance requirements.
In U.S. Pat. No. 5,615,732, a rotor having a semi-modular construction is described. The rotor is fabricated from a combination of shop assembled sector modules and field assembled components in a way to eliminate the double plate diaphragms of the normal modular rotors and thereby maximize the internal volume of the preheater which is available for heat transfer surface. The shop assembled modules comprise one or more sectors, depending primarily on the rotor size, with the field assembled components fitting between spaced shop assembled modules. The semi-modular rotor may further eliminate the stay plates, substituting support gratings which extend between the diaphragms and form open supports on which the baskets are supported. The baskets are loaded into the sectors radially, instead of axially. This eliminates the need for gaps around the baskets and the undersizing of the baskets. These support gratings are part of the shop assembled modules and also are a part of the field assembled components which actually facilitate the field assembly.
Although the semi-modular rotor construction maximizes the space available for heat transfer surface, such construction requires that a significant amount of field-erection time be dedicated to assembling half of the rotor structure and installing half of the heat-transfer surface. The use of field-labor increases the cost of the air preheater, since field-labor is more expensive than shop-labor. In addition, field-labor cannot assemble components as precisely as shop labor due to the absence of shop fixtures. Accordingly, there is an increased risk of assembly related errors which require later rework.
Briefly stated, the invention in a preferred form is a semi-modular rotor module for an air preheater having a circular rotor. The rotor module comprises a lug assembly including a lug front and a lug back spaced radially outward from the lug front. The lug front and lug back each extend longitudinally from a first end to a second end. The lug assembly also includes a cold end lug member mounted to the first ends of the lug front and the lug back and a hot end lug member mounted to the second ends of the lug front and the lug back. The lug front, lug back, cold end lug member and hot end lug member define circumferentially spaced first and second sides of the lug assembly. A single main diaphragm extends radially from an inner end portion to an outer end portion, with the inner end portion being mounted to the lug front on the first side of the lug assembly. The outer end portion is mounted to a rotor shell which is spaced radially outward from the lug back. At least one intermediate diaphragm extends radially from an inner end portion, mounted to the lug back at a position intermediate the first and second sides of the lug assembly, to an outer end portion mounted to the rotor shell.
Preferably the rotor module includes multiple intermediate diaphragms mounted in circumferentially spaced relationship to the lug back. The inner end portion of the intermediate diaphragm circumferentially spaced furthest from the main diaphragm is mounted to lug back at the position intermediate the first and second sides of the lug assembly.
The lug front and the lug back are each rectangular plates having an arcuate shape, with the lug front and lug back being substantially coaxial. Similarly, the cold and hot end lug members each have substantially coaxial arcuate-shaped outboard edges.
The lug assembly also includes multiple circumferentially spaced stiffening plates, with each stiffening plate extending longitudinally between the cold end lug member and the hot end lug member and radially between the lug front and the lug back. The longitudinally opposite edges of the stiffening plates are mounted to the cold and hot end lug members and the radially opposite edges are mounted to the lug front and the lug back. The lug assembly also includes multiple longitudinally spaced stiffening members, with each of the stiffening members extending circumferentially between adjacent stiffening plates and radially between the lug front and the lug back. The circumferentially opposite edges of the stiffening members are mounted to the stiffening plates and the radially opposite edges of the stiffening members are mounted to the lug front and the lug back.
The rotor module also comprises cold end and hot end seal extensions extending longitudinally from the cold end and hot end lug members. The rotor module further comprises multiple radially spaced stay plate assemblies which extend circumferentially between the diaphragms.
It is an object of the invention to provide an air preheater rotor which is more easily assembled from multiple rotor modules.
It is also an object of the invention to provide a lower-cost rotor structure which requires less field-erection time.
It is further an object of the invention to provide an air preheater rotor having improved interchangeability of the basketed heat exchange element.
Other objects and advantages of the invention will become apparent from the drawings and specification.